1. Field of the Invention
The present invention relates to a transfer element detect device which is used in image forming apparatus such as a copying machine, a facsimile, a laser printer and the like to detect whether a transfer element such as copying paper or the like is present at a given position in a transfer element attracting member or not.
2. Description of the Prior Art
Generally, in image forming apparatus such as a copying machine, a facsimile, a laser printer and the like, after a toner image is formed on a given image carrier, the toner image is transferred to the surface of a transfer element such as copying paper or the like. Normally, this transfer is executed in the following manner. That is, the transfer element is supplied onto a transfer element carrier, the transfer element is delivered to a given transfer position along with the movement of the transfer element carrier, and the toner image is transferred onto the transfer element. After then, the transfer element is delivered to a given peel-off position, where the transfer element is peeled off the transfer element carrier. If the respective operations are not performed normally, then there arise some troubles such as the jammed transfer element and the like. In order to avoid such troubles, the behavior of the transfer element on the transfer element carrier must be monitored and, as soon as anything abnormal occurs, it must be detected and a necessary countermeasure against it must be taken.
In view of the above, conventionally, there have been made a large number of proposals on a device which is used to detect whether the transfer element is present or not. For example, in Unexamined Japanese Patent Publication 58-46366, there is disclosed a jam detect device with a wound photosensitive drum which includes a light emitting element and a light receiving element and can detect the jammed transfer element in accordance with a ratio between the output obtained when the transfer element is present and the output when the transfer element is absent. Also, in Unexamined Japanese Patent Publication 62-185656, there is disclosed a device which includes a first light receiving element positioned in an area where paper is present, and a second light receiving element positioned in an area where no paper is present, and is arranged to judge that the paper is present when a level difference between the photo-electric conversion outputs of the first and second light receiving elements exceeds a given value.
In the above conventional devices, the previously determined outputs respectively obtained when the photosensitive element is detected and when the transfer element is detected are stored, and the jammed transfer element is detected based on the ratio between them; and, whether the transfer element is present or absent is judged based on both of the value of the output of a light sensor for transfer element attracting means and the output value of a light sensor for the transfer element attracting means. Therefore, when the whole surface of the transfer element attracting means is stained or is deteriorated uniformly, then the presence or absence of the transfer element can be detected effectively. However, in fact, it is quite rare for the transfer element attracting means to be stained or deteriorated uniformly. It is usual that the stain of the transfer element attracting means or the deterioration thereof due to damage or the like varies depending on the size of the transfer element, the bonding position of the transfer element to the transfer element attracting means, the size or the frequency of use of the transfer element, and the like. If the transfer element attracting means is stained or deteriorated ununiformly, then the quantity of light reflected from the transfer element attracting means not carrying the transfer element to the photosensor, or the quantity of the transmitted light thereof varies, with the result that the receiving light signal level also varies. Also, not only due to the stained or deteriorated transfer element attracting means but also due to the stained light receiving surface of the light sensor and due to the deteriorated elements thereof, the light receiving level also varies. Further, the light receiving signal level naturally varies depending on the kinds of the transfer elements. However, the prior art is not able to cope with such environmental variations, that is, according to the prior art, the judging standard for presence or absence of the transfer element is likely to be unstabilized, so that a sufficient detection effect cannot be provided.